X-ray Waveguides - A New Tool for High-resolution Microscopy

The waveguiding of X-rays based on front beam coupling by narrow channels has been investigated at the ESRF with the aim of creating new optics for X-ray imaging. The narrow channels are made of a low electron density rectangular polymer core embedded in a cladding of higher electron density, i.e. with a comparably lower refractive index allowing the propagation of resonant X-ray modes. At the waveguide exit, the radiation is constrained in two dimensions so that a sample may be scanned in the coherent and divergent beam, leading to a magnified holographic pattern in the detector plane. The smaller the exit, the higher the numerical aperture and hence a better spatial resolution.

A further step in reduction of the beam size and in increasing the number of photons squeezed through the channel has been made recently at beamline ID22, improving the waveguide concept. Using electron beam lithography on polymethyl methacrylate (PMMA), spin-coated onto a Si (100) wafer, a research team from the University Göttingen and ESRF created PMMA-filled channels in silicon of 30 nm x 70 nm cross-section and 4 mm length. Figure 1 shows (a) a schematic cross section with several channels of differing spacing and (b) a scanning electron micrograph of one channel. This device was illuminated by the ID22 undulator beam, pre-focussed by Kirkpatrick-Baez (KB) mirror optics of 12.5 keV. The team achieved proper beam transmission through the channels (Figure 1c), counting up to 3.5 x 106 photons/s after one channel, corresponding to a net flux density gain of the combined KB and channel system of about 4000. The radiation is emitted with a cross-section of 25 x 47 nm2 (FWHM) into a cone of about 3 mrad. The authors modelled radiation propagation in the channel using the Helmholtz wave equation and found good agreement of the beam shape with the experiment. Figure 2 shows the simulation results displayed in false colours and demonstrates that the beam consists of a few resonant X-ray modes only. This qualifies the device as a highly spatially coherent quasi-"point" X-ray source, especially useful for the holography of biological objects. X-rayed objects could be reconstructed numerically from the recorded far-field diffraction pattern at a resolution given by the waveguide dimensions.

Fig. 2: Schematic of the experimental setup: the 2D waveguide structure is aligned in the focus of a Kirkpatrick-Baez mirror system. The focused beam is coupled into the front side of the waveguide and propagates in the WG core, as illustrated by the numerical simulation of the device. The overillumating parts of the beam are damped in the absorbing cladding. Only the waveguided beam, without side peaks, spurious reflected, or primary beams, is extracted at the exit of the structure.